This invention relates to processes and systems for producing elemental sulfur. In particular, it relates to recovering elemental sulfur from acid gas using an extension of the modified Claus process.
The modified Claus process is widely used by the industry for the production of elemental sulfur. The process is designed to carry out the Claus reaction: ##EQU1## The reaction is favored by low temperature and by removal of elemental sulfur vapor.
In the conventional modified Claus process, the operating conditions of the reactors in which the Claus reaction is carried out are selected to maintain elemental sulfur in the vapor state. Otherwise, the elemental sulfur would deposit on the catalyst and deactivate it. To assure high conversion, the reaction is carried out in a plurality of consecutive reactors. Elemental sulfur is condensed and removed from the effluent of a preceding reactor before it is passed to a subsequent reactor. The removal of sulfur allows the reactors to be maintained at progressively reduced temperatures.
Often, environmental regulations require sulfur recoveries higher than those attainable with the conventional modified Claus process. Several alternatives are available for attaining these higher recoveries. For example, the tail gas from a conventional two-catalytic reactor Claus unit can be further treated by a conventional SCOT or Beavon process. These processes attain high recoveries, often well in excess of those required by regulatory agencies; however, their capital expenditures and operating costs are quite high.
An alternative to such processes is known as the Cold Bed Adsorption (CBA) process. This process is capable of theoretical recoveries well above those attainable with a conventional modified Claus process, although slightly less than those attainable with the SCOT or Beavon process. The capital expenditure and operating costs for the CBA process are less than those for SCOT or Beavon but still high. The conventional CBA process includes a thermal reactor, two conventional catalytic reactors and two low temperature catalytic reactors, known as cold bed adsorption (CBA) reactors. The reaction in a CBA reactor is generally carried out at inlet temperatures ranging from about 250.degree.-280.degree. F. (121.degree.-138.degree. C.). These low temperatures favor the forward Claus reaction and result in continuous condensation of elemental sulfur onto the alumina catalyst. By removing this sulfur from the gas phase, Claus equilibrium is further improved. The catalyst can retain approximately half its weight in sulfur before it begins to lose activity. The sulfur condensing on the catalyst tends to deactivate it. Accordingly, a second CBA reactor is provides so that while the first CBA reactor is in the recovery mode, the second reactor is being regenerated to remove elemental sulfur and vice versa.
One of the major factors contributing to the expense of the CBA process are the reactors. The elimination of one of the reactors from the CBA process would significantly reduce both overall capital expenditures and operation costs, but would tend to lower recovery to unacceptable levels.
A number of processes have been designed in an attempt to achieve acceptable recovery at lower costs. As an example, Delta Engineering Corporation's MCRC Process uses three catalytic reactors. The first catalytic reactor operates above the dew point of sulfur throughout the process. One of the remaining two reactors is operated below the dew point of sulfur (low temperature reactor) while the other reactor is being regenerated. When the catalyst in the low temperature reactor becomes deactivated due to sulfur deposits, the inputs to the two reactors are switched so that the catalyst in the reactor which was operated at a low temperature is regenerated by the high temperature gas and the reactor with the regenerated catalyst therein is operated at a temperature below the dew point of sulfur. The reactor operating in the regeneration mode is fed with a gas stream from the heat exchanger after the first catalytic reactor. This heat exchanger must be of large area and must operate at high temperature in order to supply the necessary heat for regeneration. This is, of course, an expensive equipment item. Claus equilibrium in the reactor being regenerated is much poorer than in a conventional second position Claus reactor. As a result, large concentrations of H.sub.2 S and SO.sub.2 reach the subdewpoint reactor causing it to load excessively with elemental sulfur before the second position reactor can be adequately regenerated. This high loading reduces catalyst activity and allows entrainment of elemental sulfur from the CBA bed. Accordingly, a coalescer is added downstream of the CBA although even with this enhancement acceptable recoveries may still be unattainable. The coalescer also adds to the overall cost of the system and further diminishes the savings achieved by the elimination of a catalytic reactor.
Another prior art process which attempts to achieve acceptable recovery levels at a cost comparable to a conventional CBA process is the Maxisulf system of Davy McKee AG. The Maxisulf process provides two low temperature reactors, one of which operates as the low temperature reactor while the other is regenerated. The regeneration of the low temperature catalytic reactor is accomplished by forcing therethrough a stream of hot gas from an inline burner. The hot gas flows through the reactor in the opposite direction from the low temperature gas.
This process suffers from several drawbacks. First, the alternating directions of flow accelerate the degradation of the catalyst. Second, the process utilizes an extra burner and fuel gas, both of which increase the overall cost. Third, the process introduces a dangerous condition. If oxygen is present in the regeneration gas stream, it can sulfate and therefore deactivate the catalyst. This problem can be overcome by contacting the regeneration gas with H.sub.2 S, but such contacting would further add to the cost of the process. Fourth, the quality of the fuel gas must be carefully controlled to avoid the introduction of unburned hydrocarbons into the low temperature reactor. Otherwise, the hydrocarbon would be cracked and produce a tar, which, in turn, would coat and therefore deactivate the catalyst. Finally, the inline burner adds an additional volume of gas which must be treated and processed, and which reduces the attainable recovery level.
Thus, the prior art processes have not been entirely successful in solving the problem of reducing the overall cost of the conventional CBA process. There is therefore a long-felt and still unsatisfied need for a process and a system that would require fewer reactors than the conventional CBA process, but still achieve the high required overall recovery of sulfur without the need for a further treatment of the tail gas and without the need for additional expensive components. The present invention achieves the above-stated goal.